1. Field of the Invention
Embodiments disclosed herein relate generally to diamond enhanced inserts.
2. Background Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. When weight is applied to the drill string, the rotating drill bit engages the earth formation and proceeds to form a borehole along a predetermined path toward a target zone.
There are several types of drill bits, including roller cone bits, hammer bits, and drag bits. The term “drag bits” (also referred to as “fixed cutter drill bits”) refers to those rotary drill bits with no moving elements. Fixed cutter bits include those having cutting elements attached to the bit body, which predominantly cut the formation by a shearing action. Cutting elements used on fixed cutter bits may include polycrystalline diamond compacts (PDCs), diamond grit impregnated inserts (“grit hot-pressed inserts” (GHIs), or natural diamond. Roller cone rock bits include a bit body adapted to be coupled to a rotatable drill string and include at least one “cone” that is rotatably mounted to a cantilevered shaft or journal as frequently referred to in the art. Each roller cone in turn supports a plurality of cutting elements that cut and/or crush the wall or floor of the borehole and thus advance the bit. The cutting elements, either inserts or milled teeth, contact with the formation during drilling to crush, gouge, and scrape rock at the bottom of a hole being drilled. Hammer bits are typically include a one piece body with having crown. The crown includes inserts pressed therein for being cyclically “hammered” and rotated against the earth formation being drilled.
Depending on the type and location of the cutting elements on a drill bit, the cutting elements perform different cutting functions, and as a result, also experience different loading conditions during use. Two kinds of wear-resistant inserts have been developed for use as cutting elements on drill bits: tungsten carbide inserts (TCIs) and polycrystalline diamond enhanced inserts (DEIs). Tungsten carbide inserts are typically formed of cemented tungsten carbide (also known as sintered tungsten carbide): tungsten carbide particles dispersed in a cobalt binder matrix. A polycrystalline diamond enhanced insert typically includes a cemented tungsten carbide body as a substrate and a layer of polycrystalline diamond (“PCD”) directly bonded to the tungsten carbide substrate on the top portion of the insert. A working layer formed of a PCD material can provide improved wear resistance, as compared to the softer, tougher tungsten carbide inserts.
The layer(s) of PCD conventionally include diamond and a metal in an amount of up to about 30 percent by weight of the layer to facilitate diamond intercrystalline bonding and bonding of the layers to each other and to the underlying substrate. Metals employed in PCD are often selected from cobalt, iron, or nickel and/or mixtures or alloys thereof and can include metals such as manganese, tantalum, chromium and/or mixtures or alloys thereof. However, while higher metal content typically increases the toughness of the resulting PCD material, higher metal content also decreases the PCD material hardness, thus limiting the flexibility of being able to provide PCD coatings having desired levels of both hardness and toughness. Additionally, when variables are selected to increase the hardness of the PCD material, typically brittleness also increases, thereby reducing the toughness of the PCD material.
Although the polycrystalline diamond layer is extremely hard and wear resistant, a polycrystalline diamond enhanced insert may still fail during normal operation. Failure typically takes one of three common forms, namely wear, fatigue, and impact cracking. The wear mechanism occurs due to the relative sliding of the PCD relative to the earth formation, and its prominence as a failure mode is related to the abrasiveness of the formation, as well as other factors such as formation hardness or strength, and the amount of relative sliding involved during contact with the formation. Excessively high contact stresses and high temperatures, along with a very hostile downhole environment, also tend to cause severe wear to the diamond layer. The fatigue mechanism involves the progressive propagation of a surface crack, initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling or chipping. Lastly, the impact mechanism involves the sudden propagation of a surface crack or internal flaw initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling, chipping, or catastrophic failure of the enhanced insert.
External loads due to contact tend to cause failures such as fracture, spalling, and chipping of the diamond layer. Internal stresses, for example thermal residual stresses resulting from the manufacturing process, tend to cause delamination between the diamond layer and the substrate or the transition layer, either by cracks initiating along the interface and propagating outward, or by cracks initiating in the diamond layer surface and propagating catastrophically along the interface.
The primary approach used to address the delamination problem in convex cutting elements is the addition of transition layers made of materials with thermal and elastic properties located between the ultrahard material layer and the substrate, applied over the entire substrate protrusion surface. These transition layers have the effect of reducing the residual stresses at the interface and thus improving the resistance of the inserts to delamination.
Transition layers have significantly reduced the magnitude of detrimental residual stresses and correspondingly increased durability of inserts in application. Nevertheless, basic failure modes still remain. These failure modes involve complex combinations of three mechanisms, including wear of the PCD, surface initiated fatigue crack growth, and impact-initiated failure.
It is, therefore, desirable that an insert structure be constructed that provides desired PCD properties of hardness and wear resistance with improved properties of fracture toughness and chipping resistance, as compared to conventional PCD materials and insert structures, for use in aggressive cutting and/or drilling applications.